Under Siege is a tactical/real time strategy game of squad management and combat set in a medieval fantasy world. The player controls their squads in real time, and earns gold by killing enemy units or prizes littered about the levels. Individual squads have persistent XP meters for kills as well. The more you use a squad, the faster it levels up. Units lost are gone for good! Before a battle, the player can spend gold to replace individual soldiers lost from a squad, or to upgrade a squad's abilities. Overall, the gameplay is similar to the old Bungie game "Myth".

The single player campaign has the player slowly get more unit slots in their army, and learns the abilities and variety of squads available. Multiplayer is 1v1 and pits two armies the players custom create, against each other.